[FIXED] How prevent to buy one item by two customers

Issue

I'm selling some stuff on my website using paypal IPN + PDT. I always check how many items are in the stock before payment. After customer makes his order, he is redirected into paypal for make payment. When payment is completed I'm updating database about selling stuff, until this moment I had no problem becouse I was selling unlimited items. I'm not sure what should I do in case where I have for example 10 items for sell, for example in this scenario:

There is last item for sell. Customer makes his order and he is going to pay on the paypal but he didn't payed yet (website still showing that there is one item for sell ), meanwhile customer B see that there is one item left in stock so he is going to buy it too. in this scenario both customers will pay but only one (faster one) get his product.

What I must do to dont make this happen ?

Solution

I can't give you a code answer without seeing yours first, but here's how it could be done on paper :

• When customer A clicks the "Pay" button, the item quantity is updated in the database
• If the payment goes through, do nothing, if the payment doesn't go through (cancelled, incorrect payment informations, etc) then revert the quantity back to the original

This should work fine as long as you do a final "quantity check" when the user clicks the checkout button. Let's say that both customer A and customer B have an item X in the cart, and there's only one left. They can't both pay for the item, since your system would check the quantity before redirecting to Paypal, and as soon as one of the customers do click on the button, the quantity is updated to reflect the purchase.

Answered By - Drown

Answer Checked By - Gilberto Lyons (PHPFixing Admin)